OP Young Thunder: East Meets West
by Kakusei
Summary: When Kai Harper is sent to the world of One Piece by a dying Ninja, he finds himself in a battle in the shadows to control the fate of the world! Join our Ninja-in-Training as he and the Straw Hats deal with Ninjas, new fights, and his own demons! Book 1 of the series!
1. Chapter 1

**Alright, I've mulled over this idea for a long time. I finally got an idea I wanted to do. I'm going to do my best to keep this character how I want to write him and have him act consistently. And hopefully I can make a story that enthralls you and excites you readers! Yes, this is a "Guy falls into One Piece" story. Yes, he's going to be getting some powers and act like a person who got thrown into a world he wanted to.**

Kai: Would you just get onto the story already?

Alright, alright. Just get ready to get your ass handed to you.

Kai: HEY! That wasn't part of the deal!

The deal has been altered. Pray I do not alter it further.

Kai: …you did that just for the Star Wars quote, didn't you?

**ANYWAY, I do not own One Piece, nor do I own any characters other than the Original Characters I will be creating, like Kai here.**

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><p><span>Unwanted Inheritance!<span>

Kai, a New Shinobi?!

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><p>You know how in some stories the plot manages to find the eventual hero and forces them to make a choice that ultimately helps shape them into a hero? Like in Spider-Man, where he lets that one guy go and he ends up killing Uncle Ben? Or how like in FullMetal Alchemist, the whole Human Transmutation bit causes Ed and Al to figure out how to get their bodies back to normal? Well… I wish I got a choice in the matter.<p>

I was biking back from the library, having just dropped off a copy of a volume of Ranma ½ I had read at least 20 times over. I blink for a second, and I see a guy in what looks like a classic black ninja outfit crossed with a Naruto cosplay. But the thing that stood out even more was the fact he was missing an arm that had been obviously torn off as well as having a nice gash in his torso so deep I could see his heart beating. And he was bleeding heavily.

"Holy crap, are you okay?" I asked, running over to him as I dropped my bike. I helped him lie down against a nearby tree and fished out my phone. "Lemme call 911, they hopefully can…"

"No… I'm too far…" he said in a raspy, tired voice. It was pretty obvious to me he was straining to talk. "You… you'll have to do."

_What is he talking about?_ I ignored him and started to dial, but he placed his remaining hand on my phone, pushing it down before I could hit the call button.

"You… will take my place," he replied, reaching into his cut shirt. He pulled out what appeared to be a small black box with a gold Kanji on it that I wasn't able to identify. "Idaiumi… needs you."

"…Idaiumi?" I echoed, looking at the box as he thrust it into my hands, forcing me to drop my phone. "What's this for?"

"He… will teach you," He replied, coughing for a brief moment as he pulled out a small coin with a shuriken pattern on it and another kanji on it as he threw it into my face. As it plinked off my head, I saw a flash of light as I quickly passed out.

_What the hell kinda drugs did this man just slip me?_

**-X-**

I woke up to the sound of cannonfire, or at least what I thought was that.

"Urgh… is there a Civil War reenactment going on?" I asked to no one in particular as I looked around. _This… is not where I left my bike._ All around me I saw wooden buildings like you'd see in Europe. You know, the ones that they don't tear down because unlike in America. They just build out instead of tearing stuff down, historical relevance and all that. "Okay… where in the nine levels of Hell am I?"

"_**Idaiumi."**_

"Who said that?!" I demanded to know, my head twisting from side to side to find the origin of the voice. It was then I realized I still had the black box that dying guy forced onto me. "This thing… what is it?"

"_**I am Folding Karakuri Sashimaru,"**_ the voice replied, as I managed to focus on the box again. The box started lifting from my hand and I jumped back in shock. The box seemed to open itself and inside was a few pieces as the entire thing started folding itself into a new shape. Soon the box had stopped its shifting, ending in the form of a small humanoid that reminded me of Super Sentai mechas. _**"I apologize for the abrupt introduction, but you were the only person my former Master and I could turn to."**_

"Would you explain to me what the hell is going on here?!" I snapped back at the small robot. "Some random guy I try to help shoves me a box and then flicks a coin at my head, sending me to god knows where?!"

"_**You actually just summed it up quite well,"**_ Sashimaru replied, rubbing his chin.

"I KNOW THAT! IT JUST HAPPENED TO ME!" I snapped back. "Can you tell me the other side?!"

"_**What happened to the manners of youth?"**_ Sashimaru pondered to himself, obviously ignoring me. "_**I will not speak another word until you apologize."**_

"WHAT?! I… ARGH!" I groaned, feeling very frustrated at this robot thing. "Can you at least tell me where I am now?!" Sashimaru stayed silent and seemed to turn away from me. "You gotta be kidding me." He continued to stay silent. "Fine… My name is Kai Harper, and I'm sorry I was so rude to you. Can you forgive me for being rude while I was upset?"

"_**That's better,"**_ Sashimaru replied. _**"And I can see why you would be upset. However, without you, this world would be in danger. It is up to you to help guide this world along its destined path."**_

"Okay, you're going to have to explain that one to me," I replied. "Because I don't get this destined path stuff."

"_**In short, this world's path has been led astray in secret,"**_ Sashimaru replied, _**"by the Invasion Shinobi Corps."**_

"Shinobi… you mean Ninjas?" I queried. "You gotta be kidding me, you're telling me you brought me to some random place to fight freaking NINJAS?!"

"_**That is exactly why you have been brought here,"**_ Sashimaru replied.

"I CAN'T FIGHT NINJAS!" I snapped. "I'VE NEVER EVEN BEEN IN A REAL FIGHT!"

"_**Fret not young one, I will train you in the ways of Ninpo,"**_ Sashimaru explained. _**"However, time is of the essence. I can already sense a slight disturbance."**_

I groaned, feeling this was going to be a long trip. "Okay, can you at LEAST please tell me where we are right now Sashimaru? I'd like to know what world you want me to save."

Sashimaru looked at me with a slight happy gleam in his… eyes? They looked more like jewels. _**"Very well, Genin,"**_ he replied. _**"We are in the world the Dimensional Shinobi Corps has categorized as Idaiumi, but to your world, it is known as the world of One Piece."**_

I froze. "One… One Piece? You mean we're on the freaking Grand Line, where anything can kill me right now?!" I objected.

"_**No, we're not on the Grand Line,"**_ Sashimaru revealed, giving me a brief moment of relief. _**"We're in the East Blue, on the Organ Islands, specifically the village of Orange Town. Currently Buggy the Clown is stationed here."**_

"O-Orange Town?! And Buggy the Clown is here NOW?!" I demanded in a slight frantic manner. "I don't stand a chance against Buggy! Hell, even Mohji could kick my ass right now!"

"_**I wouldn't say that," **_Sashimaru replied, looking into the sun for a brief moment, then back to me. _**"But for now, Zoro is running late. You will need to stall for time."**_

"Wait, Zoro is… Shit, that means that Nami and Luffy…"

"_**Are in danger,"**_ Sashimaru said, completing my sentence. _**"Follow me. We'll get to the Drinker's Pub quickly."**_

"Right!" I said, not thinking about the implications of my actions. Sashimaru pulled a large Shuriken and threw it, quickly jumping on his projectile and using it to fly. I quickly dashed after him, watching the small ninja-bot lead me down alleyways and streets. _I've got a gut feeling Sashimaru isn't lying. Hell, I just saw him transform and talk to me before my eyes. Can't be dreaming about that… right?_

**-X-**

Alright, that seems like a good place to stop.

Kai: What, don't wanna write the fight yet?

You wanna get your butt kicked yet?

Kai: ...continue.

Alright, thanks for reading! Please Rate, review, and favorite!


	2. Chapter 2

**Alright, time for chapter two. Now we can get into the nitty-gritty and start the action.**

Kai: Still think I'm gonna get my ass handed to me?

**Who's the writer here?**

Kai: Point taken.

**Anyway, do your lines so we can get on with this.**

Kai: *sigh* …the writer of this story does not own One Piece or any of its characters. He only owns me, Kai Harper, and any other Original Characters that he writes up to kick my ass. …HEY!

**And on with the show!**

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><p>Kai's Grand Entrance!<p>

The Brown-Eyed Ninja Appears!

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><p><strong>-X-<strong>

I ran as fast as my legs would take me, following the flying ninja robot in front of me. No, I'm not making that up. This robot, Sashimaru, was to be my new guide and mentor in the ways of the Ninja. Again, I'm not kidding. We quickly came upon the Drinker's Pub where I could easily hear the party from the ground.

"Sashimaru, how are you so sure Zoro is running late?" I asked my robotic companion.

"_**A divination Shinpijutsu,"**_ he replied. _**"It allowed me to discern the change in the timeline. However, it is not 100% reliable until the change takes hold. It seems Zoro's fate has been changed by outside means, and his sense of direction will take him a longer route."**_

"Of course it is," I groaned. "So how do you suggest I get up to the roof?"

"_**I have a tool for you to use,"**_ Sashimaru replied, producing a small scroll and unrolling it, "_**as well as a weapon or you."**_ He made a generic ninja hand sign and in a puff of smoke, he seemed to summon up what looked to be a set of small hooks on metal bars with cord behind the metal, as well as a straight-bladed, rather plain-looking black katana. I later realized that there were two Kanji on the scroll before the smoke, and after, they were gone.

"I think I've seen these before," I commented, picking up the hooked bars. "These are Cat Claws, right?"

"_**Exactly,"**_ Sashimaru replied. _**"They will let you more easily and quietly scale the side of the building. And this Katana is a tool issued by the Dimensional Shinobi Corps. We call it the Soul Mirror Blade, Hanshamaru. It will be your weapon, and ally."**_

"What do you mean by that?" I asked, slipping the sheathed blade over my shoulder and tying it onto my back.

"_**Hanshamaru's form varies from Shinobi to Shinobi,"**_ Sashimaru explained as he rolled up the scroll and stored it. _**"It will take details that reflect your inner self, and I will later teach you a Jutsu that will allow you to further draw out its power. For now, it only needs to be your blade."**_

"So… kinda like Zanpakuto from Bleach?" I continued to ask, sliding the Cat Claws onto my hands, the claws now facing away from my palms.

"_**Yes, exactly like that,"**_ he replied. _**"We found the design intriguing, and Oetsu Nimaiya allowed us to observe his method to make our own."**_

"Wait, you ACTUALLY got this thing by deriving it from-"

"_**We may discuss history later,"**_ Sashimaru interjected. _**"For now, you must ascend."**_

"Oh, right," I replied, looking at the side of the building as I latched into the wooden wall with my new climbing tools.

**-X-**

A quick but slightly tiring climb later, I was peeking over the edge of the roof with Sashimaru beside me. The party was in full swing and I could see Nami talking to Luffy.

"We made it just in time," Sashimaru commented. "I will revert to my hidden form and advise you from there."

"Wait, what?" I asked. "You're not going to help me with fighting?"

"I am only allowed to advise you," the robot ninja replied. "It is my role, as yours is to fight and protect. Never forget where we have chosen to be."

"That's… kinda confusing," I admitted as Sashimaru did his hand-sign again and folded back up into the black box, attaching himself to my belt. I resumed watching the party as Buggy approached Nami and began to loudly talk to her and Luffy. Still couldn't hear what they were saying due to the loud music and general raucous from Buggy's men.

"Alright men, load the special Buggy Ball!" Buggy ordered as he threw his head back into the air. I watched as the men scrambled for their large cannon, one scrawny guy with a round head holding a red cannonball with Buggy's pirate mark on it. The small group loaded and prepped the cannon quickly, and the saluted Buggy.

"Buggy Ball Cannon ready, Captain Buggy," the pirate stated, Buggy's smile growing slightly.

"Alright, show them!" Buggy roared. "Show them its flashy power!" The crew quickly pointed the cannon at a line of buildings, and fired the cannon.

**KABOOM-BOOM-OOM-OOM! **The cannon ripped through the houses like a finger through wet tissue paper. And as my ears were ringing, I saw Nami's face go white a bit as Buggy's crew cheered and Buggy went on a speech. His crew quickly repositioned the cannon aiming at the caged Luffy.

"Now Nami, prove your loyalty to me," Buggy said as I finally got my hearing back. "Prove it by blowing your former boss to smithereens!"

"Wh-what?" Nami asked, very shocked. "Me… kill him? Captain Buggy… that's not necessary! Let's just go drink some more and forget about that loser…"

"DO IT," Buggy ordered, his face twisting to a stern frown. The crew began chanting "Shoot! Shoot! Shoot!" as the blood boiled, hungry for Luffy's death. Nami froze in thought, contemplating what she should do. However, Buggy quickly snapped her out of it. "NAMI! Stop stalling and light the cannon!"

"Your hand is shaking," I heard Luffy call out to Nami. "That's what you get for going up against pirates without a strong will."

"A strong will?" Nami echoed, sweating a bit in anxiety. "You mean being ready to kill someone in cold blood? Is that what a pirate's strong will is?"

"No, it's not," Luffy replied calmly. "It's being ready to give your life for what you believe in. That's what a pirate is prepared to do." Nami was so engrossed in Luffy's speech that she didn't even notice one of the pirates sneaking up behind her.

"Hey, new girl, stop wasting your time," he said, snatching the matchbox away from Nami. "Guess you don't know how to do it, huh?" In his half drunk state, he didn't care to notice Nami reaching for her staff as he lit a match. "Just gotta light the match… and put it up to the cannon fuse…" Nami managed to clock him over the head, but she wasn't fast enough, as the match did its job.

"TRAITOR! After I welcomed you into my crew!" Buggy yelled as Nami quickly turned away.

"This isn't about you!" Nami barked to Luffy. "I did that because of my own feelings. No matter what I won't become a cruel pirate. YOU PIRATES TOOK SOMEONE DEAR TO ME AND I'LL NEVER BECOME WHAT I HATE!"

"Oh, so that's why you don't like pirates…" Luffy mused, before he noticed the burning fuse. "GAH! THE FUSE IS LIT!"

"You made a fool out of me!" Buggy barked. "KILL HER PAINFULLY!"

Nami however, took the time to notice the fuse as Buggy's crew charged her, and I could tell Zoro was nowhere in sight. She doused the fuse with her own hands and Buggy's crew bared down on her.

_Now!_ I quickly dashed out, using the Cat Claws to catch two of the mooks unaware, deflecting their swords away before I delivered a swift kick, sending them flying into their airborne allies.

"Who the heck is that?!" Buggy demanded as Nami turned to see me, a look of relief passing her face as she realized I had intercepted Buggy's crew for her.

"Thanks for the save… but who are you?" she asked.

"Don't thank me yet, we still have to get out of here," I replied, sliding the Cat Claws off and tying them to my belt. "As for who I am… Kai Harper, a passing-through Shinobi!"

"A Shinobi… so he's a ninja?" Nami mused, as Luffy's face went starry-eyed.

"A Ninja! That's so cool!" he cheered from his cage, "I gotta get him to join my crew!"

"A Ninja? I don't care if he's a Ninja or a Pirate, he'll be dead all the same!" Buggy stated. "Kill him anyway!"

As Buggy's crew charged me, I felt a rush. It felt like… excitement. "Alright… if you're my partner, let's do this," I said, slowly grabbing the hilt of my blade. "Hanshamaru!" I quickly drew the katana blade out, finding the hand-guard had changed shape, now resembling a pentagon with an inlaid flower pattern. "Bring it!"

The pirates came upon me as I intercepted a blade from one, catching and deflecting it before jabbing him twice and kicked him away. Another tried to stab me with two daggers but I stopped him, catching his arms with my own. I threw him into another pirate and then cut down another. However, I got blindsided by one with a club and he knocked me aside.

"Urgh… that hurts like hell," I groaned as the pirate chuckled a bit as he slowly walked towards me again.

"Lookie here, I'm gonna make a ninja pancake…" the pirate joked as he pulled back for a big smash. In my dazed state, I couldn't do anything but brace for impact. However, I soon heard the pirate groaning as to my grateful eyes, I saw Zoro using Wado Ichimoji as a club to knock out the pirate.

"ZORO!" Luffy cheered from his cage.

"Thanks for the save…" I said, catching my breath and standing up again.

"Thanks for watching my idiot captain for me," Zoro returned, resting his sheathed sword on his shoulder, "Looks like he got into a real mess."

"Me and him both," I groaned, looking at Luffy. The pirates quickly started murmuring about how Zoro was the pirate hunter.

"This is your idea of fun huh? Getting carried away by a bird and then I find you locked up in a cage?" He called out to Luffy.

"So you're Roronoa Zoro, huh?" Buggy queried as he stared at the former Pirate Hunter. "You must have come here to capture me."

"Actually, I gave up Pirate Hunting," Zoro admitted. "I have no interest in you."

"Well, I have an interest in you," Buggy retorted. "Killing you would be a great honor."

"Not interested," Zoro replied. "I don't feel like killing you today."

"Get 'im Captain Buggy!" the Buggy Pirates cheered. "Show him what you're made of!"

"Get ready Pirate hunter," Buggy said as he prepped his daggers, licking one, "Cause I'm about to see the color of your blood."

"Don't say I didn't warn you," Zoro replied as he brandished his blades.

"DIE FLASHILY!" Buggy said as he charged Zoro. His stance was wide open, even I could see that. With one attack, Buggy was in pieces.

"That was too easy," Luffy commented from his cage.

"Way too easy," I replied, slipping on one of the Cat Claws.

"Well, he certainly talked big…" Zoro replied, sheathing his blades.

"Oh my god…" Nami commented at Buggy's apparent death. She then noticed the Buggy Pirates all laughing. "They're just laughing…"

"This won't open without the key," Zoro replied. "I can't cut the iron bars."

"That's true," Luffy replied.

"Hey, hand over the key already," Zoro ordered to the now roaring-in-laughter Buggy Pirates. I saw a quick gleam behind Zoro and jumped a bit.

"Zoro!" I exclaimed as my hand shot out to try to intercept the knife.

"URK!" Zoro said, feeling something metal stabbing into his side. He looked down and saw blood starting to stain his Haramaki.

"ARGH!" I heard a familiar annoying clown yell out. Nami, Luffy, and Zoro all looked to see Buggy's disembodied hand being poked by my Cat Claws, which hadn't caught the knife fast enough to stop Buggy from stabbing the swordsman. I still managed to stop the wound from going all the way through him though. "Give me my hand back bastard!" Buggy ordered, his other parts recombining back where he was cut down.

"Sorry, I think you need a little time out," I replied, keeping a grip on his hand. "I'll give this back when you've had some time to think about what you just did."

"Why you!" Buggy retorted, throwing a dagger at me. I smirked and threw up his hand, causing it to take the dagger for me. "HEY!" Buggy replied as his unhurt hand went back to him. "What the hell is your problem?!"

"That was pretty smart," Luffy observed from his cage.

"Thanks…" Zoro panted, clutching his wound. "That could have been bad…"

"Consider us even," I replied coolly. "You stopped me from being a pancake."

"He… he's alive!" Nami replied, looking at Buggy.

"Of course I'm alive, I ate the Chop-Chop Fruit!" Buggy replied; now back to his menacing façade. "I'm a Chop-Chop Man; that means I can't be hurt by a sword!"

"He's got a Devil Fruit?!" Nami asked. "I thought those were just a myth!"

"A Chop-Chop Man? That's just silly," the resident Rubber-Man, Luffy, commented.

"But you can still be poked with needles just fine," I quipped, showing the Cat Claw I used to snatch his arm. "Looks like you're not as invincible as you say."

"QUIET YOU!" Buggy roared at me. "I'll PERSONALLY make sure you meet a flashy end!"

"Zoro, be careful, he didn't hit any vitals, but that's still a bad wound," I advised, keeping an eye on Buggy.

"STABBING SOMEONE IN THE BACK," Luffy yelled to Buggy, "THAT'S FIGHTING DIRTY, YOU BIG-NOSE!"

Buggy, his crew, and Nami all froze at the same time. "That idiot!" Nami commented, "That's the one thing you never…"

"YOU DARE TO CALL ME BIG-NOSE?!" Buggy snapped at Luffy. His hand shot off at Luffy, dagger at the ready. **WHAP!** Luffy managed to catch the dagger in his teeth!

"I swear…" Luffy said through grit teeth, his bite managing to actually break the dagger, "I'M GONNA KICK YOUR ASS! _NO ONE_ HURTS MY NAKAMA!"

"Kick MY ass?" Buggy echoed. "Bwahahahahaha! And just how do you expect to do that? You're all about to die!"

"Zoro, run for it!" Luffy ordered.

"What?! He came here to rescue you, and you want him to abandon you?!" Nami queried, obviously confused. Zoro was confused for a bit, until he got a look in Luffy's eyes.

"Got it," Zoro replied, still feeling a bit weak.

"Mind if I jump in?" I asked. "I can run interference."

"I'd be grateful," Zoro replied, taking a brief look at the cannon. Nami looked confused and scared as I turned to her.

"Just stay where you are," I replied as I held my Hashamaru up. "You'll get what we're doing."

"Like I'm going to let you leave!" Buggy retorted, aiming his arms at me. "CHOP-CHOP CANNON!" I managed to block the attacks with the Cat Claw and Hashamaru as Zoro ran behind me and grabbed the cannon. With one heave, Zoro flipped the cannon, giving me the signal to jump behind the cannon's barrel.

"AAAH! THE CANNON'S POINTED AT US!" one of the Buggy Pirates pointed out.

"And it's still got a Buggy Cannonball in it!" Buggy realized as Nami saw what we were up to.

"Glad you got it, girl," I commented as Nami quickly lit the fuse.

"NO, STOP!" Buggy pleaded as the cannon went off. **KRACK-KA-KOOM!** The Buggy Ball exploded out and hit the ground, making a very nice view-obscuring cloud.

"Not how I wanted to make a smokescreen, but that works," I replied, smiling.

"Now is a good time to get going," Zoro said, looking at us. "So who're you, girl?"

"I… I'm a thief," Nami answered.

"She's our Navigator!" Luffy chimed in.

"I never agreed to that!" Nami snapped, "and besides, shouldn't you be trying to get out of that cage?!"

"Oh right, good idea," Luffy replied.

"Luffy, don't worry about that," Zoro replied with an evil smirk on his face. "You stay right there."

"DAMMIT! I'm not gonna let those flashy bastards escape!" Buggy shrieked from within the dust cloud.

Zoro gave a heave and grunted as he started to lift the cage. "Zoro, stop that!" Luffy said to the ailing swordsman. "If you keep going your stomach will pop out!"

"If it's gonna pop out then let it!" Zoro groaned as he managed to get the cage off the ground. "Don't tell me how to do what I want!" He hoisted the heavy cage onto his shoulder, his wound bleeding faster and faster. "Don't butt in by saying anything!"

Nami looked on as they vanished into the distance. "Aren't you going with them?" Nami asked.

"I should be asking that, but I think you have other plans," I replied as she looked into the dust cloud. She turned to me only for a moment before she dashed in. She came out not a moment later with something in her hands.

"You better run before Buggy gets you," she replied.

"Need a traveling companion?" I suggested, smiling at the orange-haired beauty.

"…sure," she replied, dashing off as I followed her.

**-X-**

**I told you you'd be getting your ass handed to you.**

Kai: Did you have to make it hurt so much? I think I lost a few brain cells…

**It'd be an improvement.**

Kai: HEY!

Luffy: Hey, who's that?

Kai: Our jerkass writer. He mostly tends to speak up at the beginning and end of the chapter.

Luffy: So he's a mystery voice?

Kai: -_-; Sure, let's go with that.

**Anyway, please rate, review, and favorite!**


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